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Tag Archives: ubuntu

It turns out that there is no standard “apt” command which lists where a package was installed from. You may need this information if you have added additional APT repositories to your Debian/Ubuntu installation. I see a lot of questions at the forums (1, 2, 3, 4) and the proper solution tends to be “parse apt-cache output yourself”. Here is my solution which is very similar to this one:

The default cron job daemon on Debian and Ubuntu does not support per-user timezones (see crontab(5) man page).

Here is a solution which runs hourly cron tasks in the timezone which you specified. For example, if you want to run “test.sh” at 5 AM and 7 PM in timezone Europe/Sofia, you need to create the following “cron.hourly” script:

This article will demonstrate how to export a raw block storage device over Internet in a secure manner. Re-phrased this means that you can export a hard disk from a remote machine and use it on your local computer as it was a directly attached disk, thanks to iSCSI. Authentication and secure transport channel is provided by an SSH tunnel (more info). The setup has been tested on Ubuntu 11.10 Oneiric.

Server provisioning

Amazon AWS made it really simple to deploy a server setup in a minute:

Create an EBS volume in the same availability zone. Attach it to the EC2 instance as “/dev/sdf” (seen as “/dev/xvdf” in latest Ubuntu versions).

(optionally) Allocate an Elastic IP address and associate it with the EC2 instance.

Note that you can lower your AWS bill by buying a Reserved instance slot. Those slots are non-refundable and non-transferrable, so shop wisely. You can also stop the EC2 instance when you’re not using it and you won’t be billed for it but only for the allocated EBS volume storage.

You can use any other dedicated or virtual server which you own and can access by IP. An Amazon AWS EC2 instance is given here only as an example.

Once you have the iSCSI block device volume attached on your local computer, you can use it as you need, just like it was a normal hard disk. Only it will be slower because each I/O operation takes place over Internet. For example, you can locally encrypt the iSCSI volume with TrueCrypt, in order to prevent the administrators of the remote machine to be able to see your files.

There are situations when a friend is in need of Linux help, and the only way for you to help them is to log in to their machine and fix the problem yourself, instead of trying to explain over the phone all the steps to your friend.

Such a problem has two sub-problems:

The remote machine must accept incoming connections and provide you with shell access. The obvious way to achieve this is an SSH daemon. Many Desktop Linux distributions don’t install an SSH server by default though, for security reasons. Setting up an SSH server in this moment is slow, and could even not be possible, if your friend messed up with the packaging system, for example. So we need to find an easy way to bind a network shell on the remote machine.

We must be able to connect to the remote machine. Usually desktop machines are protected behind a firewall or NAT, and we cannot connect to them directly. If this is not the case for you, you can skip this step and just connect to the remote machine IP address. A common approach to overcome this problem is that the remote machine connects to a machine of yours, which has an accessible real IP address and has a running SSH server. Most Desktop Linux distributions have an SSH client installed by default. So all you need to do is quickly and temporarily set up an account with password authentication for your friend on your machine. Then let them log in there which will create a reverse tunnel back to their machine.

Bind a shell

Another useful tool which is usually available on Linux is the Netcat, the Swiss-army knife for TCP/IP. In order to bind a shell using the Netcat version available on Ubuntu/Debian, you need to execute the following:

I got this awesome idea from a user comment. I only extended it a bit by adding “2>&1” which redirects the STDERR error messages to the remote network client too.

Once the above has been executed on the remote machine, anyone can connect on TCP port 6000, assuming that there is no firewall. Note that you have to connect via Netcat again. A connection via Telnet adds an additional “\r” at every line end, which confuses Bash. If you need to perform actions as “root” on the remote machine, the shell needs to be executed as “root”:

The ssh client has the ability to forward a local port (review Reversing an ssh connection for a detailed example). Once you’ve set up an account for your friend, you ask them to connect to your machine:

ssh -R 6000:127.0.0.1:6000 $IP_OF_YOUR_MACHINE

Once your friend has connected to your machine, you can connect to theirs using the reverse SSH tunnel by executing the following:

nc 127.0.0.1 6000

The connection to 127.0.0.1 on TCP port 6000 is actually forwarded by SSH to the remote machine of your friend on their TCP port 6000.

Note that once you disconnect from the “nc” session, the Netcat server on the remote machine exists and needs to be restarted if you need to connect again.

Windows 7 and Linux live together on the same hard disk in perfect harmony. I had Windows 7 installed first, and a few GBytes of free space at the end of the hard drive which I left unpartitioned. Here is how to install Ubuntu:

Boot from the CD, and install it. Make sure that you choose an empty partition, and also make sure that you select to install the boot loader on the Linux partition (example: on “/dev/sda3”, and not on the main MBR “/dev/sda”).

Until here you have an Ubuntu installation which you cannot boot, yet.

Here is how to configure the Windows 7 boot loader to include Ubuntu in the boot choice menu:

Download EasyBCD and install it. EasyBCD is free for non-commercial use and offers a nice GUI to edit the Windows 7 boot loader menu.

Do the following in EasyBCD — Add New Entry -> Operating Systems -> Linux/BSD:

Type: GRUB 2

Name: Ubuntu

Device: (Automatically configured)

Finally, click on “View Settings” in EasyBCD. You should see something similar to the following:

EasyBCD ships the “stage1” boot loader of GRUB2 (\NST\AutoNeoGrub0.mbr), so you don’t have to do anything else. Just reboot your Windows 7, and the boot menu should present a choice between “Windows 7” and “Ubuntu”.

A note of caution: It is highly recommended that you do a backup of your whole hard disk before you try to install Ubuntu or modify the boot loader options.

Using a PHP script in a crontab is fairly easy, as stated in the “Using PHP from the command line” documentation… Until you start to get the following warning during the execution:

No entry for terminal type “unknown”;
using dumb terminal settings.

The script works, but this nasty warning really bothers you.

Here is a sample crontab entry:

* * * * * root sudo -u www-data php -r ‘echo “test”;’

When executed, it prints the warning on STDERR.

Yes, I know I don’t need “sudo” here, but this was my initial usage pattern as I discovered the problem, and at the first time I suspected that “sudo” got crazy. Well, it wasn’t “sudo” to blame, but PHP.

Here is the fixed crontab entry:

* * * * * root sudo -u www-data TERM=dumb php -r ‘echo “test”;’

The issue was encountered on an Ubuntu 10.04 server. I though crond usually sets $TERM to something… Anyway, problem solved.